@Ron, we are able to login from the main site, but when subsite owners attempt to login to their site at mappeddomain.com/wp-login.php it fails. It just reloads the page. Even as superadmin, I wasn't able to login to subsite's with my account via the subsite login page.
I've tested this on 3 different multisite networks and the result is the same on all 3. I've had to downgrade to 0.5.4.3 to keep things running smoothly.